Hot sprue valve assembly for an injection molding machine

ABSTRACT

A hot sprue valve assembly is provided for controlling flow of molten material through a hot sprue bushing of an injection molding machine. The valve assembly is particularly designed for use with an injection molding machine for molding centrally apertured record discs, such as video information discs. The valve assembly includes a poppet valve movable between an open position to allow flow of molten disc-forming material into a mold cavity, and a closed position to prevent flow of the disc-forming material into the mold cavity. The poppet valve is designed to mold a central aperture in the disc upon movement to the closed position, and the valve assembly is designed to accommodate air ejector apparatus for removing a solidified record disc from the machine.

This is a division of application Ser. No. 202,838, filed Oct. 31, 1980,now U.S. Pat. No. 4,340,353.

BACKGROUND OF THE INVENTION

This invention relates generally to injection molding machines and, moreparticularly, to injection molding machines including means forselectively opening and closing the hot sprue of a machine moldingassembly designed for molding centrally apertured record discs, such asvideo information discs.

An example of an injection molding machine of this particular type isdisclosed in copending and commonly assigned patent applications U.S.Ser. No. 847,367, filed in the name of J. R. Holmes et al., and entitled"METHOD AND MEANS FOR REPLICATING CENTRALLY APERTURED VIDEO DISCRECORDS", and U.S. Ser. No. 031,205, filed in the name of J. R. Holmes,and entitled "MOLDING APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORDDISCS". The apparatus described in these applications comprises aninjection molding machine having a molding assembly with first andsecond mold halves reciprocally movable between a closed position,wherein molten disc-forming material of plastic or the like is injectedinto an annular mold cavity to form a video information disc, and anopen position wherein the mold parting line is opened and the resultantmolded video information disc is removed from the machine. The annularmold cavity is defined by a pair of planar, disc-shaped stamping diescarried by platens which are in turn reciprocally movable toward andaway from each other along with the respective mold halves.

In injection molding machines of this general type, it is well known toinject molten plastic material under pressure through an injection spruebushing and into the mold cavity defined by the two disc-shaped stampingdies. When the mold cavity is filled with the molten plastic material,the surrounding machine structure including the respective mold halvesand the sprue bushing are cooled by a circulating liquid coolant toreduce the temperature of the molten plastic material and therebysolidify the material. In this manner, the molded record disc is quicklysolidified within the mold cavity whereupon the solidified disc can beremoved from the machine and the cycle restarted, thereby allowing arapid production rate of the record discs. However, a substantial amountof plastic material within the sprue bushing has a relatively largethickness compared with the plastic material within the mold cavity,whereby the plastic material within the sprue bushing takes the longesttime to solidify. The cycle time for molding the record discs istherefore dependent upon the cooling time of the sprue material, and notupon the cooling time of the disc per se. Moreover, this sprue materialwhen solidified comprises wasted plastic material which must be removedfrom the molded record disc, as by means of a punch assembly or thelike, to yield the desired thin, substantially planar record discincluding the required central aperture.

Formation of the central aperture by means of a conventional punchassembly of the type disclosed in the above-referenced copendingapplications poses a variety of problems in the production of the recorddiscs. For example, the punch step requires a certain cycle time toperform, thereby prolonging the cycle time for each record disc.Moreover, the use of a punch assembly to form the apertures aftersolidification of the record discs results in aperture shapes having aconcentricity dependent upon the state of wear of the punch assembly. Aspunch assembly wear progresses, the likelihood of the creation of debrisin the vicinity of the aperture increases, and such debris candeleteriously affect the quality of subsequently molded record discs.Finally, the use of a punch assembly occasionally results in cracking ofthe discs in the region of the aperture, whereby the discs must berejected as scrap.

Some attempts have been proposed to mold an aperture in the record discprior to solidification of the disc-forming material, and thereby avoiduse of a punch assembly. See, for example, copending U.S. applicationSer. No. 202,824 filed Oct. 31, 1980 now U.S. Pat. No. 4,372,741, Feb.8, 1983, filed in the name of A. Cane et al. and entitled "HOT SPRUEVALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE". However, theaperture-molding valve assembly of this application still requires asolidified portion of the disc-forming material with the region of theformed aperture to be removed from the machine as wasted scrap material.

The present invention provides an injection molding machine including animproved mold assembly particularly designed for use in makingrelatively thin, substantially planar record discs, such as videoinformation discs, wherein the disc production rate is independent ofthe solidification time of molten plastic material in the sprue bushingregion of the injection machine. The present invention fulfills thisneed by providing a valve assembly for isolating from the mold cavitythe molten plastic material within the sprue bushing duringsolidification of the plastic material in the mold cavity while at thesame time eliminating waste of any plastic material within the apertureregion of the disc.

SUMMARY OF THE INVENTION

The present invention is embodied in a improved injection moldingmachine for use in producing centrally apertured record discs, such asvideo information discs. The machine includes a mold assembly havingfirst and second mold halves reciprocally movable with respect to eachother between a closed position wherein molten disc-forming materialsuch as a molten plastic material is injected into an annulardisc-shaped mold cavity to form a molded record disc, and an openposition wherein the mold parting line is opened and the molded recorddisc can be extracted from the machine. At least one surface of theannular mold cavity is defined by a planar, disc-shaped stamping diecarried by one of the two molds halves for use in imparting appropriateinformation to the surface of the molded disc.

According to the invention, the molding assembly for the machineincludes a hot sprue bushing through which molten disc-forming materialis maintained at an elevated temperature and is injected into the moldcavity. This hot sprue bushing defines a flow path positioned generallyat a right angle to the plane of the mold cavity and aligned axiallywith the central axis of the mold cavity. The molten disc-formingmaterial is thus injected into the mold cavity along an axis coincidentwith the central aperture to be formed in the molded record disc.

A hot sprue valve assembly is mounted on the machine generally in axialalignment with the hot sprue bushing and includes a poppet valve foropeining and closing the flow path in the hot sprue bushing to flow ofthe molten disc-forming material. This poppet valve is biased by aspring to a normal position seated upon the bushing to prevent flow ofthe molten disc-forming material into the mold cavity. However, when thematerial is placed under the influence of an injection pressure, thepoppet valve is moved to an open position to allow the moltendisc-forming material to flow into and fill the mold cavity.

An hydraulic ram is positioned on the side of the mold cavity oppositethe sprue bushing, and is operable to advance toward the mold cavity toengage the poppet valve when the mold cavity is filled with the moltendisc-forming material. The hydraulic ram bears against the poppet valveto return the valve to a closed position seated upon the sprue bushing.Importantly, this movement of the poppet valve occurs prior tosolidification of the disc-forming material so that a central apertureis molded into the record disc having a shape corresponding to the shapeof the poppet valve.

After the disc-forming material within the mold cavity has solidified,the mold parting line is opened with the valve spring maintaining thepoppet valve in its seated or closed position. An air ejector assemblyadjacent the sprue bushing is supplied with air under pressure toair-eject the solidified molded record disc from the machine.

In the preferred embodiment of the invention, the poppet valve iscarried by a valve stem received within the sprue bushing. This valvestem is formed to include guide means engaging the interior wall of thebushing to assure accurate guided movement of the poppet valve withrespect to the axis of the sprue bushing flow path. Importantly theseguide means further define axially extending flow passages forrelatively free passage of the molten disc-forming material through thesprue bushing flow path and into the mold cavity.

Other features and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings which illustrate, by way example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a simplified cross sectional view of an injection moldingmachine including a hot sprue valve assembly embodying the presentinvention;

FIG. 2 is an enlarged fragmented cross sectional view illustrating thehot sprue valve assembly in an open position;

FIG. 3 is an exploded perspective view illustrating a portion of thevalve assembly, with portions broken away; and

FIG. 4 is an enlarged fragmented cross sectional view illustratingejection of a molded record disc from the machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and particularly to FIG. 1, there isshown a molding apparatus 10 for use in combination with an injectionmolding machine (not shown) to produce a centrally apertured record disc12, such as a video information disc. One suitable molding machine inwhich the molding apparatus 10 can be used comprises a 375 ton modelmanufactured by the Stokes Division of Pennwalt Manufacturing Co.

The molding apparatus 10 comprises a first mold half 14 including a baseplate 16 and a carrier plate 18, and a second mold half 20 including abase plate 22 and a carrier plate 24. The first and second mold halves14 and 20 along with their associated base and carrier plates 16, 18 and22, 24 are reciprocally movable toward and away from each other to openand close a relatively thin, substantially planar mold cavity 25 inwhich the record disc 12 is molded. More specifically, the mold halves14 and 20 are movable between a closed position with their carrierplates 18 and 24 in abutting relation with each other as shown in FIG.1, and an open position separated from each other (not shown in FIG. 1).A plurality of guide pins 26, one of which is shown in FIG. 1, aresecured to the first mold half 14 and received slidably within bushings28 in the second mold half 20 to maintain the two mold halves in closeaxial alignment with each other.

The carrier plates 18 and 24 of the two mold halves 14 and 20respectively include annular plate-shaped recesses 32 and 34 facingtoward each other generally in axial alignment for receiving annularplatens 36 and 38, respectively. As illustrated, these platens 36 and 38can be secured in position within the recesses 32 and 34 as by means ofbolts 44, or the like. The platens 36 and 38 respectively carryremovable support plates 37 and 39 which in turn carry disc-shapedstamping dies 40 and 42 having a surface configuration representative ofthe information to be cast into the record disc 12, and which cooperatewith each other when the mold halves 14 and 20 are in the closedposition to define the disc-shaped mold cavity 25 in which the disc 12is injection molded. Annular rings 46 and 48 can be provided about thecircumference of the mold cavity 25 for securing the support plates 37and 39, and the stamping dies 40 and 42 in fixed positions upon theirrespective platens 36 and 38, all in a well-known manner. If desired,annular center clamps can also be provided for securing the stampingdies in position, such as the lower center clamp 52 shown in FIG. 1. Thespecific construction and operation of these various components isdiscussed in further detail in copending and commonly assigned U.S.application Ser. No. 031,205, filed Apr. 18, 1979, by John R. Holmes andentitled "APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORD DISCS", thedisclosure of which is incorporated herein by reference.

An injector assembly 54 is secured by a bolt 51 or the like to the baseplate 16 of the first mold half 14, and this injector assembly 54comprises a generally cylindrical collar 55 or block having a centralinlet port 56 through which molten disc-forming material such as amolten plastic material is injected under pressure. More specifically,an injector gun assembly (not shown) supplies molten disc-formingmaterial in an appropriate quantity and under appropriate pressure forpassage through the inlet port 56 and further to the mold cavity 25.This injector gun assembly is generally conventional in the art, andthus it is not shown or described in detail herein.

The collar 55 of the injector assembly 54 includes an internal flow path57 for receiving the molten disc-forming material injected through theinlet 56. As illustrated in FIGS. 1-3, the collar 55 has a lower endwhich is secured to the enlarged upper end of a hot sprue bushing 58 asby means of at least one bolt 60. The sprue bushing 58 includes aninternal flow path 62 aligned with the injector assembly flow path 57and extending downwardly into communication with the mold cavity 25.Importantly, as shown in FIGS. 1 and 2, the injector assembly and spruebushing flow paths 57 and 62 are together oriented generally at a rightangle to the plane of the mold cavity 25, and generally in alignmentwith the axial center of the mold cavity 25. Accordingly, the moltendisc-forming material is supplied centrally to the mold cavity 25 andflows from the sprue bushing 58 in a radially outwardly direction tofill the mold cavity 25. The molten disc-forming material therebyconforms to the specific geometry of the mold cavity as defined by thetwo stamping dies 40 and 42 which include appropriate surfacediscontinuities representative of the prescribed information, such asvideo and/or audio information, to be imparted to the molded recorddisc.

The invention of this application comprises a hot sprue valve assembly64 for controllably opening and closing the sprue bushing flow path 62to flow of molten disc-forming material, and thereby control flow of themolten disc-forming material into the mold cavity 25. The valve assembly64 includes a poppet valve 66 positioned generally at the end of thesprue bushing 58 adjacent the mold cavity 25 and movable to a closedposition seated upon the sprue bushing 58. In this position, the poppetvalve 66 isolates the molten disc-forming material with the spruebushing flow path 62 from the mold cavity 25 so that the disc-formingmaterial within the mold cavity can be solidified rapidly andsubstantially independently of molten disc-forming material within thesprue bushing 58. Moreover, the poppet valve 66 advantageously operatesto mold a central aperture 68 of a desired size and shape in the disc12, as will be described.

As illustrated in FIGS. 1-3, the poppet valve 66 has a generallycircular cross sectional shape and includes a conically tapered rearvalve seat face 70 for seated engagement upon a matingly shaped valveseat 72 defined by the adjacent end of the sprue bushing 58. The poppetvalve 66 is formed integrally with a valve stem 74 which extendsupwardly, as viewed in FIGS. 1 and 2, within the sprue bushing flow path62 along the central axis thereof. This valve stem 74 is constrained foraccurate sliding, guided movement along the axis of the flow path 62 bya plurality of radially outwardly extending arms 76. These arms 76 eachhave a substantial axial length and each has a carefully selected shapeand contour for close sliding engagement with the interior wall surfaceof the sprue bushing 58. Moreover, these arms 76 are angularly spacedfrom each other to define axially extending openings 77 to allowsubstantially free and unrestricted passage of the molten disc-formingmaterial through the flow path 62 in the sprue bushing.

The upper end of the valve stem 74 includes a second plurality ofradially outwardly extending arms 78 (FIG. 3) which are angularly spacedfrom each other to define axially extending openings 79 to allowsubstantially free and unresricted passage of the molten disc-formingmaterial. These latter arms 78 terminate at their radially outwardextent in a guide cylinder 80 sized for sliding reception within thesprue bushing flow path 62. This guide cylinder 80 projects upwardly, asviewed in FIGS. 1 and 2, into an enlarged portion 63 of the flow path 62at the upper end of the sprue bushing 58 where the guide cylinder 80 issecured in a suitable manner to a radially outwardly projecting annularflange 84.

The flange 84 is in turn secured as by screws 82 to a second,similarly-sized flange 86 of a valve stem cap 88. This latter flange 86is received within a lower enlarged portion 59 of the injector assemblyflow path 57. The valve stem cap 88 has a cylindrical shape for slidingreception within the narrow upper portion of the flow path 57, and isformed integrally with an upper extension 90 of smaller cross sectionprojecting upwardly along the axis of the flow path 57. The valve stemcap 88 includes axially extending flow openings 89 for substantiallyfree and unrestricted passage of molten disc-forming material in adownward direction through the injector assembly flow path 57 to theopenings 79 formed by the arms 78 and the guide cylinder 80 of the upperend of the valve stem 74.

Both the valve stem cap 88 and the valve stem 74 are hollow forrespective reception of heaters 92 and 94, such as electrical resistanceheater cartridges. These heaters 92 and 94 are designed to maintain thetemperture level of the molten disc-forming material as it flows throughthe injector assembly 54 and the sprue bushing 58 to the mold cavity.Appropriate electrical current is supplied to the heaters 92 and 94 byconductors 96 and 98 which extend outwardly from the injection assembly54 through the cap flange 88 for suitable connection to an electricalpower supply (not shown).

A relatively large spring 100 is positioned annularly about the guidecylinder 80 of the valve stem 74 within the upper enlarged postion 63 ofthe sprue bushing flow path 62. The spring 100 comprises a compressionspring reacting between the flange 84 at the upper end of the guidecylinder 80 and an upwardly and axially presented shoulder 102 for thesprue bushing 58 within the enlarged flow path portion 63. Since thesprue bushing 58 is rigidly secured with respect to the first mold half14, the spring 100 urges the flange 84 along with the entire valveassembly 64 in an upward direction as viewed in FIGS. 1 and 2. Thismoves the valve stem 74 upwardly within the sprue bushing flow path 62to correspondingly move the poppet valve 66 to a normal seated positionupon the sprue bushing valve seat 72.

The arms 76 about the stem 74 combine with the guide cylinder 80 and thevalve stem cap 88 to constrain the entire valve assembly 64 foraccurately guided axial motion with respect to the axis of the spruebushing flow path 62. Conveniently, a cap sleeve 99 is received over thetwo flanges 84 and 86 for sliding movement within the enlarged portions59 and 63 of the flow paths 57 and 63. This cap sleeve 99 is secured tothe flanges 84 and 86 by the screws 82, and axial rotation of the entiresubassembly is prevented by a guide pin 101 received respectively withinslots 103 and 105 in the cap sleeve 99 and the injector assembly 54.

In operation of the machine, molten disc-forming material is injectedunder pressure through the injector assembly inlet 56 for passagethrough the flow paths 57 and 62 to the mold cavity 25. The pressure ofthe injected molten material is sufficient to overcome the force bias ofthe spring 100 and move the poppet valve 66 off the seat 72 to an openposition. Such movement of the poppet valve 66 allows the moltendisc-forming material to flow radially outwardly from the poppet valve66 into the mold cavity 25 to fill the mold cavity. The heaters 92 and94 maintain the disc-forming material within the flow paths 57 and 62 ina molten state at all times.

An hydraulic ram 104 is positioned on the side of the mold cavity 25opposite the sprue bushing 58 to return the poppet valve 66 to a closedposition seated upon the valve seat 72 when the mold cavity 25 is filledwith the disc-forming material. More specifically, the hydraulic ram 104is carried by the second mold half 20 within a guide sleeve 106 fixed inposition with respect to the carrier plate 24 of the second mold half20. This guide sleeve 106 is positioned in axial alignment with thesprue bushing 58 on the opposite side of the mold cavity 25, andslidably carries the ram 104 for movement toward and away from the moldcavity 25.

The hydraulic ram 104 includes at its end opposite the mold cavity 25 anenlarged piston 108 positioned within a matingly configured cylindricalchamber 110 formed between the base plate 22 and the carrier plate 24 ofthe second mold half 20. Hydraulic fluid ports 112 and 114 (FIG. 2) areformed in the carrier plate 24 and the base plate 22 to supply hydraulicfluid under pressure to the upper side of the piston 108 for moving theram 104 in a downward direction away from the poppet valve 66, or tosupply hydraulic fluid under pressure to the lower side of the piston108 to move the ram 104 in an upward direction toward the poppet valve.Conveniently, as shown in FIGS. 1 and 2, the piston 108 and the ram 104are hollow for reception of a heater 116 in the form of an electricalresistance heater cartridge or the like for which electrical current isprovided as by means of a conductor 118.

The poppet valve 66 and the hydraulic ram 104 include matinglydiscontinuous surfaces for assuring accurate and guided axial alignmenttherebetween. For example, as shown, the poppet valve 66 includes adownwardly projecting lug 120 having a truncated conical shape forreception into a matingly configured recess 122 at the upper end of theram 104.

When molten disc-forming material is injected into the mold cavity 25,as described above, the hydraulic fluid under pressure is supplied tothe upper surface of the piston 108 to retract the hydraulic ram 104 toa position generally adjacent to, but not extending into, the moldcavity 25. When this occurs, the pressure of the injected disc-formingmaterial is sufficient to move the poppet valve 66 to the open positionshown in FIG. 2. The ram 104 provides a stop for axial movement of thepoppet valve 66 and helps to assure accurate axial translation thereof.

After the mold cavity 25 is filled with the molten disc-formingmaterial, the injector gun assembly (not shown) is typically operated ina known manner to reduce the pressure applied to the injecteddisc-forming material to a lower, so-called "hold" pressure for apredetermined time period. This allows the molten material to packtightly within the mold cavity 25, and thereby overcome or compensatefor effects of inherent shrinkage of the material. Importantly, however,this "hold" pressure is sufficient to maintain the poppet valve 66 inits open position of FIG. 2, while the hydraulic ram 104 is maintainedin its retracted position.

After the predetermined "hold" time period, the injector gun assembly(not shown) substantially removes injection pressure from the moltendisc-forming material. At the same time, the hydraulic ram 104 isoperated by means of pressurized hydraulic fluid supplied to the lowersurface of the piston 108 to engage and move the poppet valve 66 acrossthe mold cavity upwardly to its closed position seated upon the valveseat 72. This closed position of the poppet valve 66 is shown in FIGS. 1and 4. Importantly, the poppet valve 66 is sized and shaped tocorrespond with the desired size and shape of the central aperture 68 tobe formed in the record disc 12, whereby this movement of the poppetvalve 66 across the mold cavity functions to mold the aperture 68 intothe disc prior to solidification of the disc-forming material.

With the poppet valve 66 in the closed position of FIGS. 1 and 4, themolten disc forming material within the sprue bushing flow path 62 isisolated from the molten disc-forming material within the mold cavity25. At this time, the material within the mold cavity 25 can besolidified rapidly and substantially independently of the plasticmaterial within the sprue bushing 58. More specifically, a cooling fluidsuch as water can be circulated in heat exchange relation with the moldcavity 25 via inlets 124 and through a plurality of coolant fluid flowpassages 126 defined by the carrier plates 18 and 24 and theirrespective platens 36 and 38. This circulating coolant fluid reduces thetemperature level of the disc-forming material within the mold cavity 25and thereby solidies that material rapidly to form the molded recorddisc 12. Of course, this coolant fluid is circulated from these passages126 via an outlet (not shown) to an appropriate heat exchanger or thelike for reduction of the coolant fluid temperature and recirculationthereof to the mold assembly 10 through the inlets 124.

The hot sprue valve assembly 64 of this invention thus offerssignificant advantages over the prior art in that it is not necessary towait for the molten disc-forming material within the sprue bushing 58 tosolidify before the record disc 12 can be removed from the mold cavity25. Alternately stated, it is necessary only to solidify the materialhaving a relatively small thickness within the mold cavity 25 before thesolidified record disc 12 can be removed from the machine and asubsequent molding cycle initiated. This significantly decreases thecycle time for producing a single record disc 12, and therebysubstantially increases the overall disc production rate of theinjection molding machine.

The hot sprue valve assembly 64 of this invention is furtheradvantageous in that the cylindrical poppet valve 66 functions directlyto mold the central aperture 68 into the record disc 12 prior tomaterial solidification thereby eliminating the need for punching orother post-solidification formation of the central aperture. Thismolding technique eliminates waste of disc-forming material from theaperture region of the resultant molded disc 12, and further eliminatesany need to grind or otherwise machine the size and shape of the centralaperture to a final size. This technique also prevents possible crackingof the disc 12 or the creation of small debris during punching which canbecome trapped within the mold cavity 25 and deleteriously affectsubsequent record disc formation. Still further, the use of the poppetvalve 66 to mold the central aperture eliminates the requirement forperiodic replacement of a conventional punching apparatus which can wearover a period of time.

As soon as the disc-forming material within the mold cavity 25 hassolidified to form the molded record disc 12, the mold cavity 25 isopened in a conventional manner to expose the molded record disc 12. Thespring 100 maintains the poppet valve 66 in the closed position toprevent drooling of molten disc-forming material from the spure bushing58. When the mold halves are moved to open the mold cavity 25,pressurized air is supplied through an inlet port 130 in the first moldhalf 14 to an annular pressure chamber 132 defined between the spruebushing 58 and the associated support plate 37. The air under pressurewitin this annular chamber 132 is directed axially toward the recorddisc 12 through a plurality of slots 134 to air-eject the record disc 12from the mold halves 14 and 20. This ejection operation is illustratedin detail in FIG. 4.

The hot sprue valve assembly of this invention thus provides anaccurately controlled and guided poppet valve 66 for movement between anopen position allowing molten disc-forming material to enter the moldcavity 25, and a closed position to prevent the material from enteringthe mold cavity. The poppet valve 66 is assisted by an hydraulic ram 104upon movement to the closed position whereby the poppet valve molds thecentral aperture 68 within the disc 12 prior to material solidification.Waste of disc-forming material and the possible creation of debris orcracking of the molded record disc 12 is thereby avoided.

Various moldifications and improvements to the hot sprue valve assemblyembodiments of this invention are believed to be apparent to one skilledin the art. Accordingly, no limitation upon the invention is intended,except as set forth in the appended claims.

What is claimed is:
 1. In an injection molding machine for producingcentrally apertured record discs, said machine including a generallydisc-shaped mold cavity and a sprue bushing for flow of moltendisc-forming material into the mold cavity, a method of controlling flowof the disc-forming material into the mold cavity comprising the stepsof:positioning a poppet valve generally at the end of the sprue bushingadjacent the mold cavity; biasing the poppet valve to a normal closedposition seated upon the adjacent end of the sprue bushing to preventflow of the molten disc-forming material from the sprue bushing into themold cavity; injecting molten disc-forming material into the spruebushing at a pressure sufficient to force the poppet valve to movegenerally across the mold cavity to an open position spaced from thesprue bushing to allow flow of the molten disc-forming material throughthe sprue bushing into the mold cavity; returning the poppet valve toits closed position by advancing a ram on the side of the mold cavityopposite the sprue bushing into bearing engagement with the poppetvalve; allowing the disc-forming material in the mold cavity tosolidify; and ejecting the solidified molded disc from the mold cavity.2. The method of claim 1 including the steps of supporting the poppetvalve by a valve stem received along the axis of the sprue bushing flowpath, and guiding said valve stem for sliding movement along the axis ofthe spure bushing flow path with a plurality of angularly spaced armsprojecting radially outwardly from the valve stem.
 3. The method ofclaim 1 including the step of heating the valve stem to maintain thedisc-forming material in a molten state within the sprue bushing flowpath.
 4. The method of claim 3 including the step of heating the ram.